The Moving Picture Experts Group (MPEG) standard defines how Intensity Stereo (IS) coded audio streams are decoded and how this information is represented in the incoming coded bit stream. However, the encoder processing is not standardized. Stereo and multi-channel audio signals in MPEG-AAC usually contain channel pairs (e.g. a pair of left and right channels). If a channel pair is encoded using IS coding, only one audio channel will be transmitted instead of the pair along with gain values. The transmitted audio channel will be decoded as the left output channel of the channel pair and the right channel is derived from the left channel using applied gain values transmitted in the audio bit-stream. There is one gain value transmitted in the bit stream per scale factor band (SFB) of the audio stream.
IS coding can be turned on or off independently in each SFB and each window group. The main advantage of IS coding is the bit rate savings obtained by transmitting only one channel instead of two. However, if IS coding is applied too aggressively, audible artifacts and distortions may appear that may cause an associated image to appear more narrow, objects in the scene may appear shifted, or some objects may even disappear. To avoid distortions, IS coding must be applied to SFBs and window groups in a discreet manner.